1. Field of the Invention
The present invention relates to a CPP type magnetoresistive sensor in which a current flows in a direction perpendicular to film surfaces of layers of a multilayered film. More particularly, the present invention relates to a magnetoresistive sensor and a method of manufacturing the magnetoresistive sensor, which can effectively increase the product (ΔRA) of a resistance change amount (ΔR) and a sensor area (A), and which can more easily and reliably bring magnetization of a free magnetic layer and magnetization of a pinned magnetic layer into an orthogonal state than the related art.
2. Description of the Related Art
FIG. 13 is a sectional view of a structure of a conventional magnetoresistive sensor, as viewed from the side of a sensor surface positioned to face a recording medium.
The magnetoresistive sensor shown in FIG. 13 is of CPP (Current Perpendicular to the Plane) type in which a current is supplied in a direction perpendicular to film surfaces of a multilayered film.
Reference numeral 1 in FIG. 13 denotes a lower electrode layer. A multilayered film 6 is formed on the lower electrode layer 1, the multilayered film 6 comprising a free magnetic layer 2, a nonmagnetic material layer 3, a pinned magnetic layer 4, and an antiferromagnetic layer 5 which are successively formed in this order from the lowermost side. A track width Tw is defined by a width size of the multilayered film 6 in the direction of the track width (X-direction as shown). The free magnetic layer 2 and the pinned magnetic layer 4 are each formed of, e.g., a NiFe alloy. The nonmagnetic material layer 3 is formed of, e.g., Cu, and the antiferromagnetic layer 5 is formed of, e.g., PtMn.
On each of both sides of the multilayered film 6 in the direction of the track width, a first insulating layer 7, a hard bias layer 8, and a second insulating layer 9 are successively formed on the lower electrode layer 1 in this order from the lowermost side. The first insulating layer 7 and the second insulating layer 9 are each formed of an insulating material such as Al2O3. The hard bias layer 8 is formed of, e.g., CoPt.
Further, as shown in FIG. 13, an upper electrode layer 10 is formed so as to cover an upper surface of the multilayered film 6 and upper surfaces of both the second insulating layers 9.
In the magnetoresistive sensor shown in FIG. 13, with heat treatment performed on the sensor under a magnetic field, an exchange coupling magnetic field is generated between the antiferromagnetic layer 5 and the pinned magnetic layer 4, and hence the pinned magnetic layer 4 is fixedly magnetized in the height direction (Y-direction as shown). Also, since the hard bias layers 8 formed on both sides of the free magnetic layer 2 in the direction of the track width (X-direction as shown) are magnetized in the direction of the track width, magnetization of the free magnetic layer 2 is put in order in the X-direction as shown with a longitudinal bias magnetic field applied from the hard bias layers 8.
Thus, the fixed magnetization of the pinned magnetic layer 4 and the magnetization of the free magnetic layer 2 are in a state orthogonal to each other. Then, when a current is supplied from the electrode layers 1, 10 to flow through the multilayered film 6 in the direction of film thickness and a magnetic field leaked from the recording medium is applied in the Y-direction, the magnetization of the free magnetic layer 2 is changed from the X-direction toward Y-direction. Depending on the relationship between a variation in the direction of magnetization of the free magnetic layer 2 and the fixed direction of magnetization of the pinned magnetic layer 4, electrical resistance is changed (this is called the magnetoresistive effect). As a result, the magnetic field leaked from the recording medium is detected in accordance with a voltage change caused upon the change in value of the electrical resistance.
As described above, the magnetoresistive sensor shown in FIG. 13 is of CPP type in which a current flows through the multilayered film 6 in the direction perpendicular to the film surfaces. Because the CPP type magnetoresistive sensor can produce a larger reproduction output with a reduction of the device size than that produced by a CIP (Current in the Plane) type magnetoresistive sensor in which a current flows through the multilayered film 6 in a direction parallel to the film surfaces, the CPP type sensor is expected to be properly adaptable for the reduction of the device size, which will be necessitated with a tendency toward a higher recording density in future.
One problem to be overcome for realizing practical use of the CPP type magnetoresistive sensor adapted for a tendency toward a higher recording density is to increase the product (ΔRA) of a resistance change amount (ΔR) and a sensor area (A) in the direction parallel to the film surfaces. With an increase of ΔRA, the reproduction output can be increased more effectively.
In the case of the CPP type magnetoresistive sensor, ΔRA can be increased by increasing a film thickness t1 of the pinned magnetic layer 4. Although ΔRA can be increased by increasing the film thickness t1 of the pinned magnetic layer 4, the exchange coupling magnetic field generated between the antiferromagnetic layer 5 and the pinned magnetic layer 4 is reduced as the film thickness t1 of the pinned magnetic layer 4 increases. Such a reduction of the exchange coupling magnetic field causes the magnetization of the pinned magnetic layer 4, which should be firmly pinned in the height direction, to easily fluctuate with the external magnetic field, etc. Hence, the orthogonal relation in the direction of magnetization between the free magnetic layer 2 and the pinned magnetic layer 4 is lost, thus resulting in a deterioration of characteristics such as lowering of a resistance change rate (ΔMR) and occurrence of Barkhauzen noise.
For that reason, it has hitherto been difficult to effectively achieve an increase of ΔRA and reliable adjustment in fixing the magnetization of the pinned magnetic layer 4.
FIG. 14 shows a magnetoresistive sensor, which is also of CPP type similar to the magnetoresistive sensor shown in FIG. 13, but differs from it in a manner of controlling the magnetization of the free magnetic layer 2. In the magnetoresistive sensor shown in FIG. 13 the hard bias layers 8 are formed on both sides of the free magnetic layer 2 in the direction of the track width, and the magnetization of the free magnetic layer 2 is oriented in the direction of the track width with the longitudinal bias magnetic field applied from the hard bias layers 8.
On the other hand, in the magnetoresistive sensor shown in FIG. 14, second antiferromagnetic layers 11 are formed on the free magnetic layer 2 with a spacing corresponding to the track width Tw left therebetween in the direction of the track width (X-direction as shown). The magnetization in opposite end portions of the free magnetic layer 2 is pinned in the direction of the track width by the exchange coupling magnetic field generated between the free magnetic layer 2 and the second antiferromagnetic layers 11, whereas the magnetization in a central area of the free magnetic layer 2 is weakly put into a single domain state to such an extent that the magnetization is reversible with the external magnetic field.
Then, as shown in FIG. 14, insulating layers 12 made of, e.g., Al2O3 are formed on the second antiferromagnetic layers 11, and an upper electrode layer 10 is formed so as to cover the free magnetic layer 2 and both the insulating layers 12.
In the magnetoresistive sensor shown in FIG. 14, as in the magnetoresistive sensor shown in FIG. 13, it is also been difficult to effectively achieve an increase of ΔRA and reliable adjustment in fixing the magnetization of the pinned magnetic layer 4.
Further, in the magnetoresistive sensor shown in FIG. 14, because it must be subjected to heat treatment under a magnetic field to generate the exchange coupling magnetic field between the free magnetic layer 2 and the second antiferromagnetic layers 11, the heat treatment under the magnetic field must be performed twice, including the heat treatment under the magnetic field to generate the exchange coupling magnetic field between the pinned magnetic layer 4 and the antiferromagnetic layer 5.
However, various restrictions are imposed on conditions, such as the intensity of the magnetic field and the temperature of the heat treatment, when the second heat treatment under the magnetic field is performed to generate the exchange coupling magnetic field between the free magnetic layer 2 and the second antiferromagnetic layers 11 after performing the first heat treatment under the magnetic field to generate the exchange coupling magnetic field between the pinned magnetic layer 4 and the antiferromagnetic layer 5.
The reason is as follows. When performing the second heat treatment under the magnetic field, if the intensity of the magnetic field, for example, is greater than the exchange coupling magnetic field generated between the pinned magnetic layer 4 and the antiferromagnetic layer 5, the magnetization of the pinned magnetic layer 4, which should be pinned in the height direction, is caused to fluctuate under an effect of the second heat treatment under the magnetic field. Such a fluctuation can be avoided by reducing the intensity of the magnetic field applied in the second heat treatment under the magnetic field. However, if the intensity of the magnetic field is too much reduced, the exchange coupling magnetic field cannot be generated at an appropriate level of intensity between the free magnetic layer 2 and the second antiferromagnetic layers 11, and the magnetization in the opposite end portions of the free magnetic layer 2 cannot be properly pinned. Consequently, a magnetoresistive sensor satisfactorily adaptable for a tendency toward a narrower track cannot be manufactured.
Thus, the presence of the above-described restrictions makes it difficult to manufacture the magnetoresistive sensor, and there has been a fear of reduction in yield of manufacturing of the magnetoresistive sensor because the orthogonal relation between the free magnetic layer 2 and the pinned magnetic layer 4 is lost depending on the conditions in the heat treatment under the magnetic field.